Rotor structure for electric rotating machine

ABSTRACT

A rotor structure for an electric rotating machine has a rotor, a plurality of strip-shaped permanent magnets which are arranged in a circumferential direction of the rotor, each of which penetrates the rotor in an axial direction of the rotor, flux barriers which are air gaps provided at positions corresponding to width-direction end portions of the permanent magnets to face the width-direction end portions of the permanent magnets, and demagnetization prevention holes, each of which is provided at a position in a vicinity of a corresponding one of the flux barriers and on an opposite side of the flux barrier from a stator and configured to reduce demagnetization at a corresponding one of the width-direction end portions of the permanent magnets.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of PCT/JP2015/055378filed Feb. 25, 2015, and claims foreign priority to Japanese PatentApplication No. 2014-054265 filed Mar. 18, 2014, which are incorporatedherein by reference in their entirety.

BACKGROUND

Technical Field

The present invention relates to a rotor structure for a permanentmagnet-type electric rotating machine.

Related Art

As a rotor structure for a permanent magnet-type electric rotatingmachine, Patent Literature 1 discloses a structure in which fluxbarriers (air gaps) are provided as a countermeasure against leakagemagnetic flux at width-direction end portions of strip-shaped permanentmagnets arranged to penetrate a rotor in an axial direction thereof.

Specifically, the strip-shaped permanent magnets are arranged at aregular pitch in a circumferential direction, while penetrating therotor in the axial direction in a vicinity of an outer peripheralsurface of the rotor. The flux barriers are provided at positionscorresponding to the width-direction end portions of the permanentmagnets in the rotor and extend to positions near the outer peripheralsurface of the rotor. The flux barriers are provided to face thewidth-direction end portions of the permanent magnets.

By providing the flux barriers at the width-direction end portions ofthe permanent magnets, the width between the outer peripheral surface ofthe rotor and each flux barrier is reduced to reduce the leakagemagnetic flux (magnetic flux which is not linked to the stator and doesnot contribute to the torque), and the motor torque is improved byincreasing the magnetic flux of the magnets usable as the motor torque.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Publication No.    2008-211934

SUMMARY OF INVENTION

It is well known that, a permanent magnet generally starts to beirreversibly demagnetized and is permanently demagnetized below aknickpoint of a material of the permanent magnet, so that the magneticforce generated by the permanent magnet decreases.

Moreover, even with the structure in which flux barriers are provided atwidth-direction end portions of strip-shaped permanent magnets arrangedin a rotor as in the technology of Patent Literature 1, a permanentmagnet-type electric rotating machine tends to be irreversiblydemagnetized at a portion on a stator side of each permanent magnet,i.e., a portion near a supply side of an external magnetic field,because the magnetic flux density in the magnet becomes small at such aposition.

This is because the magnitude of the external magnetic field determineshow easily the irreversible demagnetization occurs. Portions near thestator, which is a source of generation of the external magnetic field,tend to be irreversibly demagnetized easily. Especially, width-directionend portions of strip-shaped permanent magnets are susceptible to theexternal magnetic field.

One or more embodiments of the present invention provides a rotorstructure for an electric rotating machine that may reduce thedemagnetization at width-direction end portions of strip-shapedpermanent magnets arranged to penetrate a rotor in the axial directionthereof.

A rotor structure for an electric rotating machine according to one ormore embodiments of the present invention comprises strip-shapedpermanent magnets; flux barriers provided to face width-direction endportions of the permanent magnets; and demagnetization prevention holeseach being provided at a position in a vicinity of the corresponding oneof the flux barriers and on an opposite side of the flux barrier from astator and each having a shape which allows the demagnetizationprevention hole to be close to a demagnetization region localized on astator side of the corresponding one of the width-direction end portionsof the permanent magnets.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically showing a main portion of a rotorstructure according to one or more embodiments of the present invention.

FIG. 2 is an enlarged diagram of part A of FIG. 1.

FIGS. 3A-3B are diagrams showing how the magnetic flux density isdistributed at a width-direction end portion of a permanent magnet ofFIG. 2 in comparison with a Comparative Example.

FIGS. 4A-4B are diagrams showing how the magnetic flux flows at thewidth-direction end portion of the permanent magnet of FIG. 2 incomparison with a Comparative Example.

FIG. 5 is a graph showing the relationship between the demagnetizationratio and the magnet temperature at the width-direction end portion ofthe permanent magnet shown in FIG. 2.

FIG. 6 is a graph showing the relationship between the demagnetizationratio and the distance between a stator-side edge portion and ademagnetization prevention hole at the width-direction end portion ofthe permanent magnet shown in FIG. 2.

FIG. 7 is a graph showing the relationship between the demagnetizationratio and the position of the demagnetization prevention hole at thewidth-direction end portion of the permanent magnet shown in FIG. 2.

FIGS. 8A-8C are diagrams showing modifications of arrangement ofpermanent magnets.

FIGS. 9A-9B are diagrams showing modifications of cross-sectional shapesof permanent magnets.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention are described indetail with reference to the drawings. In embodiments of the invention,numerous specific details are set forth in order to provide a morethorough understanding of the invention. However, it will be apparent toone of ordinary skill in the art that the invention may be practicedwithout these specific details. In other instances, well-known featureshave not been described in detail to avoid obscuring the invention.

FIG. 1 schematically shows a positional relationship between a rotor 2and a stator 3 arranged around a periphery of the rotor 2 with anecessary air gap provided therebetween in a permanent magnet-typeelectric rotating machine 1 according to one or more embodiments of thepresent invention.

The rotor 2 includes a laminate of multiple core single plates made of amagnetic metal. In a vicinity of an outer peripheral surface 2 a of therotor 2, multiple permanent magnets 4 are arranged at a regular pitch ina circumferential direction. The permanent magnets 4 are press fitted soas to penetrate the rotor in an axial direction.

Each of the permanent magnets 4 is formed like a strip, and has arectangular shape in a cross-section perpendicular to the axialdirection of the rotor 2. The permanent magnet 4 is magnetized in itsthickness direction (a direction in parallel with the shorter sides ofthe rectangular cross-section), and has an N pole on one of the majorsurfaces (the surfaces forming the longer sides in the rectangularcross-section) and an S pole on the other major surface. The multiplepermanent magnets 4 are arranged with the N poles and the S poles beingalternately arranged in the circumferential direction in the outerperipheral surface 2 a of the rotor 2.

In the permanent magnet-type electric rotating machine 1, magnetic flux,so called leakage magnetic flux, is generated near width-direction endportions 4 a of the permanent magnets 4. The leakage magnetic flux isnot linked to the stator 3 and does not contribute to the torque. A fluxbarrier 5 is provided at a position corresponding to each of thewidth-direction end portions 4 a of the permanent magnets 4 as acountermeasure against the leakage magnetic flux. Note that the widthdirection of the permanent magnet 4 refers to a direction perpendicularto the thickness direction and the axial direction of the rotor 2 (adirection in parallel with the longer sides of the rectangularcross-section).

Each flux barrier 5 is formed as an air gap by extending a through-hole6, into which the permanent magnet 4 is press fitted, in the widthdirection of the permanent magnet 4. The flux barrier 5 faces thewidth-direction end portion 4 a of the permanent magnet 4. In otherwords, the flux barrier 5 is arranged adjacent to the width-directionend portion 4 a of the permanent magnet 4. The width-direction endportion 4 a of the permanent magnet 4 is exposed in the air gap of theflux barrier 5. Note that an inner peripheral surface 5 a, which definesthe air gap of the flux barrier 5, has a substantially arc-like shape inthe cross-section perpendicular to the axial direction of the rotor 2.

At a base portion of the flux barrier 5, a step portion 7 is providedwhich determines the position of the permanent magnet 4 press fittedinto the through-hole 6 in the width direction as shown in FIG. 2.

In one or more embodiments of the present invention, by the provision ofthe flux barriers 5 at the width-direction end portions 4 a of thepermanent magnets 4, the magnetic path width between the outerperipheral surface 2 a of the rotor 2 and each flux barrier 5 isreduced, so that the leakage magnetic flux is reduced.

In addition, the rotor 2 is provided with demagnetization preventionholes 8 for reducing the demagnetization at the width-direction endportions 4 a of the permanent magnets 4. Each of the demagnetizationprevention holes 8 is provided at a position which is near thecorresponding one of the flux barriers 5 and on an opposite side of theflux barrier 5 from the stator 3. Here, a peripheral region of eachpermanent magnet 4 is divided into two pieces along a central plane ofthe permanent magnet 4 (along an imaginary plane passing through acenter of the permanent magnet 4 in the thickness direction and being inparallel with the two major surfaces of the permanent magnet 4). In thiscase, a region in contact with a major surface 4 b closer to the outerperipheral surface 2 a of the rotor 2 is referred to as a stator 3 sideregion, whereas a region in contact with a major surface 4 c moredistant from the outer peripheral surface 2 a of the rotor 2 is referredto as an opposite side region from the stator 3. The distance betweenthe outer peripheral surface 2 a of the rotor 2 and each of the majorsurfaces of the permanent magnet 4 can be defined as, for example, thelength of a perpendicular line drawn from a point on the outerperipheral surface 2 a of the rotor 2 to the major surface of thepermanent magnet 4 in the cross-section perpendicular to the axialdirection of the rotor 2. In one or more embodiments of the presentinvention, since each demagnetization prevention hole 8 is provided atthe position on the opposite side of the corresponding flux barrier 5from the stator 3, the demagnetization prevention hole 8 is overlappedwith the flux barrier 5 in a view of the permanent magnet 4 in itsthickness direction.

FIGS. 3A-3B show a distribution of the internal magnetic flux density ofthe permanent magnet 4 in the cross-section perpendicular to the axialdirection of the rotor 2 in magnetic flux density ranks S1 to S10. Asshown in FIGS. 3A-3B, a demagnetization region S1 is localized at anedge portion of the permanent magnet 4 on the stator 3 side, morespecifically, at a region of the width-direction end portion 4 a of thepermanent magnet 4 on the stator 3 side (a region closer to the outerperipheral surface 2 a of the rotor 2).

In this respect, the demagnetization prevention hole 8 in one or moreembodiments of the present invention has a shape which allows thedemagnetization prevention hole 8 itself to be as close to thedemagnetization region S1 as possible.

For example, as shown in FIG. 2, the demagnetization prevention hole 8is formed in an arc shape extending along the periphery of the fluxbarrier 5 in the cross-section perpendicular to the axial direction ofthe rotor 2. The demagnetization prevention hole 8 extends around thewidth-direction end portion 4 a of the permanent magnet 4 in a curvedmanner from a center side (the opposite side from the stator 3) to anouter side (the stator 3 side) of the rotor 2. In other words, thedemagnetization prevention hole 8 extends along the periphery of theflux barrier 5 from the opposite side region from the stator 3 to thestator 3 side region. In addition, a distal end portion 8 a (a portionwhich is the most distant from a center of the permanent magnet 4 in itswidth direction in terms of the position in the width direction of thepermanent magnet 4) of the demagnetization prevention hole 8 ispositioned outside a distal end portion 5 b of the flux barrier 5 interms of the position in the width direction of the permanent magnet 4.Note that the positional relationship between the flux barrier 5 and thedistal end portion 8 a of the demagnetization prevention hole 8 is notlimited to this one, but can be set, as appropriate, according to thespecifications of the rotor.

In addition, the demagnetization prevention hole 8 is present at aposition on a line segment 0 extended from the width-direction endportion 4 a of the permanent magnet 4 in parallel with a d axis. In oneor more embodiments of the present invention, the d axis coincides witha straight line connecting a rotation center axis of the rotor 2 and awidth-direction center of the permanent magnet 4. In other words, a baseend portion 8 b of the demagnetization prevention hole 8 intersects witha straight line passing through the width-direction end portion 4 a ofthe permanent magnet 4 and being in parallel with the thicknessdirection of the permanent magnet 4 in the cross-section perpendicularto the axial direction of the rotor 2.

In addition, a partition wall 9 extends between the flux barrier 5 andthe demagnetization prevention hole 8. The partition wall 9 partitionsthe flux barrier 5 and the demagnetization prevention hole 8 from eachother. The partition wall 9 constitutes a part of the inner peripheralsurface 5 a (a part on the opposite side from the stator 3) whichdefines the air gap of the flux barrier 5. A base end portion 9 b of thepartition wall 9 constitutes the above-described step portion 7. Adistal end portion 9 a of the partition wall 9 extends to a vicinity ofthe distal end portion 5 b of the flux barrier 5. A width dimension t(thickness dimension) of the partition wall 9 is substantially constantfrom the base end portion 9 b to the distal end portion 9 a of thepartition wall 9.

As described above, in the rotor 2 of one or more embodiments of thepresent invention, the demagnetization prevention holes 8, which preventthe demagnetization at the width-direction end portions 4 a of thepermanent magnets 4, are provided at positions which are in the vicinityof the flux barriers 5 provided at the width-direction end portions 4 aof the strip-shaped permanent magnets 4 and which straddle the fluxbarriers 5.

Thus, not only the leakage magnetic flux can be reduced by the fluxbarriers 5 at the width-direction end portions 4 a of the strip-shapedpermanent magnets 4 in the rotor 2, but also the reverse magnetic fieldflowing from the stator 3 into the width-direction end portions 4 a ofthe permanent magnets 4 can be reduced by providing the demagnetizationprevention holes 8 to reduce the demagnetization. This makes it possibleto further increase the magnetic flux of the magnets usable for themotor torque and improve the motor torque.

FIGS. 3A-3B shows the range of the demagnetization region S1 distributedon the stator 3 side of the width-direction end portion 4 a of thepermanent magnet 4. FIG. 3A shows that of one or more embodiments of thepresent invention, and FIG. 3B shows that of a Comparative Example inwhich no demagnetization prevention holes 8 are provided. In one or moreembodiments of the present invention, the demagnetization preventionhole 8 is provided at a predetermined position corresponding to thewidth-direction end portion 4 a of the permanent magnet 4. Hence, thearea of the demagnetization region Si in the width-direction end portion4 a of the permanent magnet 4 is smaller in one or more embodiments ofthe present invention than in the Comparative Example.

FIG. 5 shows the ratio of decrease in motor-induced voltage after anapplication of an electric current, i.e., the demagnetization ratio, ina case where the temperature of the permanent magnets 4 of the rotor 2of each of one or more embodiments of the present invention and aComparative Example is changed and a reverse magnetic field is generatedby applying an equal electric current to the stator 3. The line arepresents the demagnetization ratio of one or more embodiments of thepresent invention, and the line b represents the demagnetization ratioof the Comparative Example in which no demagnetization prevention holes8 are formed. As shown in FIG. 5, the demagnetization ratio of one ormore embodiments of the present invention is low, and takes a valuesubstantially a half of the demagnetization ratio of the ComparativeExample.

FIGS. 4A-4B show images of reverse magnetic fields in width-directionend portions 4 a of permanent magnets 4. FIG. 4A shows that of one ormore embodiments of the present invention, and FIG. 4B shows that of aComparative Example in which no demagnetization prevention holes 8 areformed. The thickness of each arrow indicates the intensity of the flowof the magnetic flux from the stator 3. A thicker arrow indicates thatthe magnetic flux in that direction is stronger. As shown in FIGS.4A-4B, the reverse magnetic field flowing from the stator 3 into thewidth-direction end portion 4 a of the permanent magnet 4 can be reducedmore in one or more embodiments of the present invention than in theComparative Example. This is because the provision of thedemagnetization prevention hole 8 increases the magnetoresistance of themagnetic flux flowing through the width-direction end portion 4 a of thepermanent magnet 4.

In one or more embodiments of the present invention, the demagnetizationprevention hole 8 is formed in an arc shape substantially along the fluxbarrier 5 in a curved manner around the width-direction end portion 4 aof the permanent magnet 4 from the center side to the stator 3 side ofthe rotor 2. This shape allows the demagnetization prevention hole 8 tobe as close to the demagnetization region 51 as possible.

This shortens the distance between the edge portion of the permanentmagnet 4 on the stator 3 side and the demagnetization prevention hole 8,so that the magnetic saturation in the above-described partition wall 9is promoted, and the anti-demagnetization performance can be improved.

FIG. 6 is a graph showing the relationship between the demagnetizationratio and the distance between the stator 3 side edge portion of thepermanent magnet 4 and the demagnetization prevention hole 8. Asdescribed above, the magnetic saturation in the partition wall 9 reducesthe reverse magnetic field flowing from the stator 3 into thewidth-direction end portion 4 a of the permanent magnet 4. Here, thecloser to the stator 3 side edge portion of the permanent magnet 4 thepartition wall 9 is, the more the magnetic saturation at that portion isfacilitated, and the greater the effect of reducing the demagnetizationis, as shown in FIG. 6.

In addition, since the width dimension t of the partition wall 9 issubstantially constant, it is possible to make the magnetic saturationsubstantially constant over the range of the bridge length of thepartition wall 9, and to increase the effect of reducing the reversemagnetic field from the stator 3. Note that, according to one or moreembodiments of the present invention, a width dimension t is smaller,because a larger effect of reducing the demagnetization can be obtained.However, an excessively small width dimension t may make it difficult topunch out pieces from a laminated steel plate. A lower limit value ofthe width dimension t according to one or more embodiments of thepresent invention is twice the thickness of a single laminated steelplate, whereas an upper limit value thereof according to one or moreembodiments of the present invention is twice the thickness of thepermanent magnet 4 (the dimension of the permanent magnet 4 in parallelwith the d axis).

In addition, each of the demagnetization prevention holes 8 is set to bepresent at a position on the line segment 0 extended from thecorresponding one of the width-direction end portions 4 a of thepermanent magnets 4 in parallel with the d axis.

FIG. 7 is a graph showing the relationship between the demagnetizationratio and the position of the demagnetization prevention hole 8. As canbe seen from this graph, the effect saturates, when the demagnetizationprevention holes 8 are present on a negative side of the line segment 0,where the demagnetization prevention holes 8 are located behind thepermanent magnets 4, i.e., when the demagnetization prevention holes 8are present inside the width-direction end portions 4 a of the permanentmagnets 4 in terms of the position in the width direction of thepermanent magnets 4. Accordingly, for improvement of theanti-demagnetization performance, according to one or more embodimentsof the present invention, each of the demagnetization prevention holes 8is placed at the position on the extension of the line segment 0.

Embodiments of the present invention are described above. However, theembodiments are mere examples provided to help understanding of thepresent invention, and the present invention is not limited to the aboveembodiments. The technical scope of the present invention is not limitedto the specific technical matters disclosed in the embodiments above,but includes various modifications, alterations, alternativetechnologies, and the like which can be easily derived from the specifictechnical matters.

For example, in one or more of the above-described embodiments, therotor structure using a single strip-shaped permanent magnet 4 permagnetic pole is shown as an example; however, one or more embodimentsof the present invention can also be applied to rotor structures usingmultiple strip-shaped permanent magnets 4 per magnetic pole as inmodifications shown FIGS. 8A-8C.

FIG. 8A shows an example in which each magnetic pole is constituted ofthree permanent magnets 4 arranged in a triangle, FIG. 8B shows anexample in which each magnetic pole is constituted of two permanentmagnets 4 arranged in a V shape, and FIG. 8C shows an example in whicheach magnetic pole is constituted of two permanent magnets 4 havingdifferent lengths in the width direction and being arranged at multiplestages in the d-axis direction. In FIGS. 8A-8C, the flux barriers 5 arenot shown for convenience.

In addition, the cross-sectional shape of each of the permanent magnets4 in the cross-section perpendicular to the axial direction of the rotor2 is not limited to the rectangular cross-section, but any shape can beselected, such as a trapezoidal shape shown in FIG. 9A, asemi-elliptical shape shown in FIG. 9B, or an unillustrated arc-likecurved shape, according to the specifications of the rotor.

One or more embodiments of the present invention may be applied to apermanent magnet-type electric rotating machine.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

REFERENCE SIGNS LIST

-   1 electric rotating machine-   2 rotor-   3 stator-   4 permanent magnet-   4 a width-direction end portion of permanent magnet-   5 flux barrier-   6 through-hole-   8 demagnetization prevention hole-   9 partition wall-   S1 demagnetization region-   0 line segment in parallel with d axis-   t width dimension of partition wall

The invention claimed is:
 1. A rotor structure for an electric rotatingmachine, comprising: a rotor; a plurality of strip-shaped permanentmagnets that are arranged in a circumferential direction of the rotor,wherein each of the plurality of strip-shaped permanent magnetspenetrates the rotor in an axial direction of the rotor; flux barriersthat are air gaps provided at positions corresponding to width-directionend portions of the permanent magnets, wherein the flux barriersrespectively face the width-direction end portions of the permanentmagnets; and demagnetization prevention holes, each of thedemagnetization prevention holes is provided at a position in a vicinityof a corresponding one of the flux barriers and each of thedemagnetization prevention holes is located on an inner side, in radialdirection, of the flux barrier, wherein the inner side is opposite anouter side, in a radial direction, of the flux barrier, wherein theouter side of the flux barrier faces a stator, wherein each of thedemagnetization prevention holes is configured to reduce demagnetizationat a corresponding one of the width-direction end portions of thepermanent magnets, wherein each of the demagnetization prevention holeshas a shape that allows the demagnetization prevention hole to be in aperipheral vicinity of a demagnetization region that is localized on aradially outer region of the width-direction end portion of thepermanent magnet, and wherein the radially outer region faces an outerperipheral surface of the rotor.
 2. The rotor structure for an electricrotating machine according to claim 1, wherein each of thedemagnetization prevention holes is formed to be present at a positionon a line segment extended from the width-direction end portion of thepermanent magnet in parallel with a d axis.
 3. The rotor structure foran electric rotating machine according to claim 1, wherein a widthdimension of a partition wall between each of the flux barriers and thecorresponding one of the demagnetization prevention holes is set to besubstantially constant.
 4. The rotor structure for an electric rotatingmachine according to claim 2, wherein a width dimension of a partitionwall between each of the flux barriers and the corresponding one of thedemagnetization prevention holes is set to be substantially constant.